System and method for measuring the linear and rotational acceleration of a body part

ABSTRACT

A system and method for determining the magnitude of linear and rotational acceleration of and direction of impact to a body part, such as a head includes positioning a number of single-axis accelerometers proximate to the outer surface of the body part. A number of accelerometers are oriented to sense respective linear acceleration orthogonal to the outer surface of the body part. The acceleration data sensed is collected and recorded. A hit profile function is determined from the configuration of the body part and the positioning of the plurality of accelerometers thereabout. A number of potential hit results are generated from the hit profile function and then compared to the acceleration data sensed by the accelerometers. One of the potential hit results is best fit matched to the acceleration data to determine a best fit hit result, which yields the magnitude of linear acceleration to and direction of an impact to the body part. The rotational acceleration of the body part can also be estimated from the magnitude of linear acceleration of and direction of the impact to the body part.

This application claims the benefit of the Provisional Application No.60/239,379, filed Oct. 11, 2000.

GOVERNMENT RIGHTS

The invention described herein was made in the course of work undergrant number 1R43HD4074301 from the National Institutes of Health. TheU.S. Government may retain certain rights in this invention.

BACKGROUND OF THE INVENTION

The present invention relates to recording of the magnitude anddirection of impact to and the linear and rotational acceleration of abody part, such as a human head, of person engaged in physical activity,such as during the play of a sport.

More particularly, it relates to a helmet based system which istypically worn while playing a sport such as football or hockey, and tothe method of recording and storing data relating to the linear androtational accelerations of the person's body part due to impact forcesacting thereon. The present invention relates also to head mountedsystems which are also worn during game play, such as a head band, thatdoes not employ helmets, such as soccer.

It should be understood that the present invention relates generally tothe linear and rotational acceleration of a body part, and mostimportantly, the head. The present invention, as will be discussed indetail below, is capable of monitoring any body part of an individualbut has particular application in monitoring the human head. Therefore,any reference to a body part is understood to encompass the head and anyreference to the head alone is intended to include applicability to anybody part. For ease of discussion and illustration, discussion of theprior art and the present invention is directed to the head of human, byway of example and is not intended to limit the scope of discussion tothe human head.

There is a concern in various contact sports, such as football andhockey, of brain injury due to impact to the head. During such physicalactivity, the head or other body part of the individual, is oftensubjected to direct contact to the head which results in impact to theskull and brain of the individual as well as movement of the head orbody part itself.

Much remains unknown about the response of the brain to headaccelerations in the linear and rotational directions and even lessabout the correspondence between specific impact forces and injury,particularly with respect to injuries caused by repeated exposure toimpact forces of a lower level than those that result in a catastrophicinjury or fatality. Almost all of what is known is derived from animalstudies, studies of cadavers under specific directional and predictableforces (i.e. a head-on collision test), from crash a dummies, from humanvolunteers in well-defined but limited impact exposures or from othersimplistic mechanical models. The conventional application of knownforces and/or measurement of forces applied to animals, cadavers, crashdummies, and human volunteers limit our knowledge of a relationshipbetween forces applied to a living human head and resultant severe andcatastrophic brain injury. These prior studies have limited value asthey typically relate to research in the automobile safety area.

The concern for sports-related injuries, particularly to the head, ishigher than ever. The Center for Disease Control and Preventionestimates that the incidence of sports-related mild traumatic braininjury (MTBI) approaches 300,000 annually in the United States.Approximately ⅓ of these injuries occur in football. MTBI is a majorsource of lost player time. Head injuries accounted for 13.3% of allfootball injuries to boys and 4.4% of all soccer injuries to both boysand girls in a large study of high school sports injuries. Approximately62,800 MTBI cases occur annually among high school varsity athletes,with football accounting for about 63% of cases. Concussions in hockeyaffect 10% of the athletes and make up 12%-14% of all injuries.

For example, a typical range of 4-6 concussions per year in a footballteam of 90 players (7%), and 6 per year from a hockey team with 28players (21%) is not uncommon. In rugby, concussion can affect as manyas 40% of players on a team each year. Concussions, particularly whenrepeated multiple times, significantly threaten the long-term health ofthe athlete. The health care costs associated with MTBI in sports areestimated to be in the hundreds of millions annually. The NationalCenter for Injury Prevention and Control considers sports-relatedtraumatic brain injury (mild and severe) an important public healthproblem because of the high incidence of these injuries, the relativeyouth of those being injured with possible long term disability, and thedanger of cumulative effects from repeat incidences.

Athletes who suffer head impacts during a practice or game situationoften find it difficult to assess the severity of the blow. Physicians,trainers, and coaches utilize standard neurological examinations andcognitive questioning to determine the relative severity of the impactand its effect on the athlete. Return to play decisions can be stronglyinfluenced by parents and coaches who want a star player back on thefield. Subsequent impacts following an initial concussion (MTBI) may be4-6 times more likely to result in a second, often more severe, braininjury. Significant advances in the diagnosis, categorization, andpost-injury management of concussions have led to the development of theStandardized Assessment of Concussion (SAC), which includes guidelinesfor on-field assessment and return to sport criteria. Yet there are noobjective biomechanical measures directly related to the impact used fordiagnostic purposes. Critical clinical decisions are often made on thefield immediately following the impact event, including whether anathlete can continue playing. Data from the actual event would provideadditional objective data to augment psychometric measures currentlyused by the on-site medical practitioner.

Brain injury following impact occurs at the tissue and cellular level,and is both complex and not fully understood. Increased brain tissuestrain, pressure waves, and pressure gradients within the skull havebeen linked with specific brain injury mechanisms. Linear and rotationalhead acceleration are input conditions during an impact. Both direct andinertial (i.e. whiplash) loading of the head result in linear androtational head acceleration. Head acceleration induces strain patternsin brain tissue, which may cause injury. There is significantcontroversy regarding what biomechanical information is required topredict the likelihood and severity of MTBI. Direct measurement of braindynamics during impact is extremely difficult in humans.

Head acceleration, on the other hand, can be more readily measured; itsrelationship to severe brain injury has been postulated and tested formore than 50 years. Both linear and rotational acceleration of the headplay an important role in producing diffuse injuries to the brain. Therelative contributions of these accelerations to specific injurymechanisms have not been conclusively established. The numerousmechanisms theorized to result in brain injury have been evaluated incadaveric and animal models, surrogate models, and computer models.Prospective clinical studies combining head impact biomechanics andclinical outcomes have been strongly urged. Validation of the varioushypotheses and models linking tissue and cellular level parameters withMTBI in sports requires field data that directly correlates specifickinematic inputs with post-impact trauma in humans.

In the prior art, conventional devices have employed testing approacheswhich do not relate to devices which can be worn by living human beings,such as the use of dummies. When studying impact with dummies, they aretypically secured to sleds with a known acceleration and impactvelocity. The dummy head then impacts with a target, and theaccelerations experienced by the head are recorded. Impact studies usingcadavers are performed for determining the impact forces and pressureswhich cause skull fractures and catastrophic brain injury.

There is a critical lack of information about what motions and impactforces lead to MTBI in sports. Previous research on football helmetimpacts in actual game situations yielded helmet impact magnitudes ashigh as 530 g's for a duration of 60 msec and >1000 g's for unknowndurations with no known MTBI. Accelerometers were held firmly to thehead via the suspension mechanism in the helmet and with Velcro straps.A recent study found maximum helmet accelerations of 120 g's and 150 g'sin a football player and hockey player, respectively. The disparity inmaximum values among these limited data sets demonstrates the need foradditional large-scale data collection.

Most prior art attempts relate to testing in a lab environment. However,the playing field is a more appropriate testing environment foraccumulating data regarding impact to the head. A limitation of theprior art involves practical application and widespread use ofmeasurement technologies that are size and cost effective forindividuals and teams. Therefore, there would be significant advantageto outfitting an entire playing team with a recording system tomonitoring impact activities. This would assist in accumulating data ofall impacts to the head, independent of severity level, to study theoverall profile of head impacts for a given sport. Also, full-time headacceleration monitoring would also be of great assistance inunderstanding a particular impact or sequence of impacts to a player'shead over time that may have caused an injury and to better treat thatinjury medically.

To address this need, there have been many attempts in the prior art toprovide a system for recording the acceleration of an individual's bodypart, such as their head. For example, prior art systems have employedtri-axial accelerometers which are affixed as a module to the back of afootball helmet. Such tri-axial accelerometers provide accelerationsensing in the X, Y and Z directions which are orthogonal to each other.Tri-axial accelerometer systems require that the accelerometers beorthogonal to each other Also, such tri-axial accelerometer systems havebeen extremely expensive making it cost prohibitive for widespreadcommercial installation on an entire team.

Prior art systems, have also attempted to precisely locate the variouscombinations of linear and rotational accelerometers, in specificorthogonal arrays, within a helmet to obtain complete three-dimensionalhead kinematics. Such arrays require that the accelerometers bepositioned orthogonal to each other. It is impractical, from a size,cost and complexity standpoint, for commercial application of sucharrays in helmet or head mounted systems.

Obviously, accelerometer arrays for measuring linear and rotationalaccelerations cannot be readily mounted inside the human head, as isdone with instrumented test dummy heads. Other sensing technologies,such as gyroscopes, magnetohydrodynamic angular rate sensors and GPSsensors, do not currently fulfill the practical and technicalspecifications for a commercially available system. Also, the use ofmulti-axis accelerometer systems placed in a mouthguard are impracticalbecause wires need to run from the helmet or backpack into the user'smouth from the power source and to a telemetry unit, which might presenta hazard to the players and limited compliance among them.

In view of the foregoing, there is a demand for a head accelerationsensing system that can be manufactured and installed at very low costto permit widespread utilization. There is a demand for a system thatcan be installed in many, many individuals, such as an entire footballteam roster of over 60 players, to provide research opportunities anddata that have not yet been available to the scientific communitybefore. Further, there is a demand for a system and method for measuringthe linear and rotational acceleration of a body part that is easy toinstall and comfortable for the individual to wear. There is also adesire to provide a low-cost system and method that can record andaccurately estimate linear and rotational acceleration of a body part.

SUMMARY OF THE INVENTION

The present invention preserves the advantages of prior art body partacceleration systems and associated methods. In addition, it providesnew advantages not found in currently available methods and systems andovercomes many disadvantages of such currently available methods andsystems.

The invention is generally directed to the novel and unique headacceleration monitoring technology that is a highly portable system thatdesigned to measure and record acceleration data in linear directionsand to estimate rotational accelerations of an individual's head anddirection and magnitude of impact during normal activity, such as duringgame play. While the present invention is specifically developed for thehead, monitoring of other body parts, or the body in general, isenvisioned and considered within the scope of the present invention.

The system and method of the present invention offers the opportunity tostudy head acceleration, human tolerance limits, the range and directionof accelerations in humans in relation to morphological features (e.g.,neck circumference, head volume, neck length), and the relationshipbetween precise measures of head acceleration in linear and rotationaldirections and acute consequence to brain physiology and function.Moreover, it provides the ability to measure an individual's cumulativeexposure to linear and rotational accelerations while allowingunaffected performance of everyday sports and activities.

The system and method of the present invention is designed as a standardcomponent of otherwise conventional sporting gear, in particular thehelmet or as an independent head mounted system. The system and methodof the present invention is designed for determining the magnitude oflinear acceleration and direction of impact to a body part as well asthe rotational acceleration of a body part, such as a head. A number,such as three, single-axis accelerometers are positioned proximal to theouter surface of the body part and about a circumference of the bodypart in a known spaced apart relation from one another. Theaccelerometers are oriented to sense respective linear accelerationorthogonal to the outer circumference of the body part. Dual-axis ortri-axis accelerometers may also be employed to provide an additionaldirection of acceleration sensing which is tangential to the surface ofthe skull of the head. Such tangential acceleration data may beoptionally employed in further analysis.

The acceleration data sensed is recorded for each accelerometer. A hitprofile function is determined from the configuration (i.e. geometry) ofthe body part and the positioning of the plurality of accelerometersthereabout. A number of potential hit results are generated from the hitprofile function and then compared to the acceleration data sensed bythe accelerometers. One of the potential hit results is best fit matchedto the acceleration data to determine a best fit hit result. Themagnitude acceleration and direction of acceleration due to an impact tothe body part are determined from applying the hit profile function tothe best fit hit result. The rotational acceleration of the body partcan also be estimated from the magnitude and direction of the impact tothe body part.

The data recorded is either recorded on a memory card or other massmemory means installed locally in the helmet, or is transmitted to anearby receiver for storage on a computer's hard drive or otherconventional mass storage device using conventional telemetrytechnology. The present invention provides storage of data over a lengthof time such that cumulative exposure effects and thus limits can beestablished for further or future participation in the sport by theindividual wearing the helmet equipped with the present invention. Thedata also allows detection of impacts to the head which precede theoccurrence of a brain injury. For this purpose the system and method ofthe present invention could be modified to record detailed data onlywhen the accelerations exceed a defined threshold. The data may beprocessed immediately as the data is recorded, or at a later time so asto integrate and otherwise determine the linear, rotational and normalcomponents of acceleration of the player's head.

The present invention is applicable for use with other parts of thebody. For instance, other applications could include the study of theacceleration of body parts in relation to each other (e.g., among polevaulters, high jumpers, or gymnasts), or to understand factors affectingacceleration in sprinters and swimmers (e.g., starting and turns).Because of its portability, small size, and convenient light weight, thesystem and associated method of the present invention can also be usedto study the acceleration of the body parts of live animals. Forexample, the acceleration and deceleration of birds in flight could bestudied with a modified version of the present invention.

It is therefore an object of the present invention to employaccelerometers arranged in a manner orthogonal to the surface of thebody part instead of arrays of accelerometers orthogonal to each other.

It is a further object of the invention to provide an inexpensive systemthat can still achieve results which are within the acceptable range oferror for the given scientific question, study or hypothesis.

Another object of the present invention is to provide a system andmethod of calculating and estimating the linear and rotationalacceleration that is easy to install and is comfortable for theindividual to wear without affecting their game play either in a helmetor head band environment.

It is yet another object of the present invention to provide a systemand method of measuring and calculating the linear and rotationalacceleration that can be installed commercially at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features which are characteristic of the present invention areset forth in the appended claims. However, the invention's preferredembodiments, together with further objects and attendant advantages,will be best understood by reference to the following detaileddescription taken in connection with the accompanying drawings in which:

FIG. 1 is a side view the system of the present invention installed in afootball helmet on an individual's head;

FIG. 2 is a top view of the system shown in FIG. 1;

FIG. 3 is a schematic top view of a head with a coordinate system shownthereon;

FIG. 4 is a perspective view of an accelerometer employed in the presentinvention

FIG. 5 is a side elevational view of a accelerometer embedded withincushioning of a football helmet;

FIG. 6 is a side elevational view of an accelerometer held in place in ahelmet by a T-shaped holder;

FIG. 7 is a diagram illustrating the wireless telemetry systemoptionally employed in the present invention;

FIG. 8 is a graphical display of the fitting of the algorithm to thecollected data; and

FIG. 9 is a graphical comparison of simulated peak acceleration andlocation of impact with ideal peak acceleration and location of impactfor two sets of accelerometer orientations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a system and method for measuring, i.e.estimating, the linear and rotational acceleration of a body part. Forease of illustration, the body part will be described below as a humanhead. Unlike the prior art, the present invention uses single axisaccelerometers orthogonal to the surface of the body part and notnecessarily orthogonal to each other to enable the estimation of boththe linear acceleration and rotational acceleration of the body part.

Referring first to FIG. 1, a side view of an installed system 10 of thepreferred embodiment of the present invention installed on body part 12,namely a human head. FIG. 2 shows a top view of this system 10 of thepreferred embodiment of the present invention. The system 10 includes anarray of accelerometers, generally referenced as 14, positioned aboutthe periphery of the skull 16 of the head 12. Preferably, an array of 3accelerometers 14 or more are located as close as possible to the outersurface of the skull 16 and arranged in the same plane which preferablypasses through the center of gravity 18 of the body part 12. However,less than three accelerometers 14 may be used and the arrangement of theaccelerometers 14 may be in different configurations around the surfaceof the skull, provided that their sensitive axis is orthogonal to thesurface of the skull. The array of accelerometers defines a band aboutthe skull 16 of the head 12.

In the preferred embodiment shown in FIGS. 1 and 2, an array of threeaccelerometers 14 a, 14 b and 14 c are provided and are positioned atknown positions about the outer periphery of the skull 16. As shown inFIG. 2 and in accordance with the coordinate system defined in FIG. 3,accelerometer 14 a is positioned at 0 degrees while accelerometer 14 bis positioned at 120 degrees and accelerometer 14 c at −120 degrees. Theuse of as few accelerometers 14 as possible to estimate linear androtational acceleration of the head 12 within a prescribed errortolerance is balanced against the cost associated of the system, namelythe added cost per accelerometer 14 and associated circuitry 15employed. If greater accuracy of the estimation of the linear androtational acceleration of the head 16 is desired, the number ofaccelerometers 14 may be increased to improve the overall “goodness offit” of the actual acceleration measurements to the estimation of linearand rotational acceleration of the head 16.

The Analog Devices ADXL193/278 family of accelerometers are preferredfor use in the system 10 of the present invention. An example of the apreferred accelerometer 14 is shown in FIG. 4. The ADXL278 is similar tothe ADXL 193 except that it is a two-axis accelerometer rather thansingle-axis. Critical specifications include: small size (4.5 mm×4.5mm×2.0 mm), low mass (1.5 g), operation at 3.3 V, high output (250 gmax), high sensitivity (27 mv/g) and low cost. One axis measuresaccelerations towards the center of the head, while the second axismeasures acceleration tangential to the surface of the head. While asingle-axis accelerometer 14 is preferred, the second axis measurementof the ADXL 278 can also provided additional acceleration informationfor further processing and analysis. This second axis includesadditional data tangential to the head during rotational experiments inthe laboratory. While the ADXL 193/278 family of accelerometers arepreferred, other accelerometers 14 may be employed to carry out thepresent invention.

In accordance with the present invention, the accelerometers 14 must beheld close to the skull 16 of the head 12 to best measure theacceleration of the head. Direct attachment of accelerometers to thehead is optimal but not feasible. Attempts to mount accelerometersdirectly to the helmet shell result in measures of helmet deformationrather than head acceleration. Variations among football helmet paddingand liners and other helmet designs for other sports demand genericmounting concepts that are universally applicable. Thus, the mounting ofthe accelerometers 14 should not alter helmet performance or protrudefrom existing internal padding more than 1 mm. Also, the accelerometers14 should be contained within and/or attached to the helmet to alloweasy removal of both the helmet or headband and the accelerometers 14.

The present invention provides a structure for maintaining theaccelerometers 14 in a position as close as possible to the skull 16while being as comfortable as possible. As shown in FIG. 5, it has beendiscovered that the preferred structure for positioning of theaccelerometers proximate to the skull is to contain the accelerometers14 within an air bladder 22 mounted within the helmet, generallyreferenced as 20.

As shown in FIG. 5, the preferred embodiment for carrying theaccelerometers is to capture the accelerometer 14 inside an air-bladder22 itself such that the pressure inside the bladder 22 will provide theforce necessary to place the accelerometer 14 in direct apposition tothe skull 16 of the head 12 when the bladder 22 is inflated. Additionalaccelerometers 14 are respectively placed in appropriately positionedair bladders 22 within the helmet 20 to provided the array ofaccelerometers as described above. In accordance with this attachmentmethod, an RF welding process can be employed to pass the requisitecabling 24 through the bladder seal without compromising the integrityof the bladder 22. A significant advantage of this method is that, for agiven padding configuration, the accelerometers 14 will be orientedsimilarly for all players using that model helmet 20.

Alternatively, as shown in FIG. 6, the accelerometers 14 may berespectively installed in a plastic T-shaped holder 26 for placing theaccelerometers 14 approximately in apposition to the skull 16 of thehead 12. Each plastic T-shaped holder 26 respectively holds anaccelerometer 14 between the cushions 22 in a football helmet and indirect apposition to the surface of the skull 16. This T-shapedaccelerometer holder 26, for example, may be constructed of Delrin andwith a 4 mm slot 28 for holding and orienting the accelerometer 14. TheT-shaped holder 26 is pressed against the skull 16 of the head 12 whenthe air bladders 22 are inflated to 20 psi, for example. This structurefor positioning the accelerometers 14 may not be preferred because it ispossible that the users could feel the accelerometers 14 pushing againstthe skull 16 of their head 12.

Also, direct attachment of the accelerometers 14 to the air bladder 22of the helmet 20 with a foam covering (not shown) is possible, althoughnot preferred, because the sensitive axis of these devices is along aplane parallel to the top of the device. The minimum dimension of theaccelerometer 14 and its mounting board 15, as shown in FIG. 4, in thatdirection is 7 mm, which caused the unit to act effectively as a pointsource against the head 12.

Still further and within the scope of the present invention, a mesh netor bandana carrying the array of accelerometers 14 units may be worn onthe head or coupled to the inside of the helmet or a multi-layer softfoam interface that captured the accelerometers between layers or aspring-loaded construct attached to the shell of the helmet 20 betweenthe foam pads (not shown) and air bladders 22.

As shown in FIG. 7, the above described array of accelerometers 14 areelectrically interconnected together to form an entire system 30 for thecollection, recording and processing of head acceleration data. Thesystem includes the accelerometers 14 in an array in a head-mountedsensor system (HMSS), generally referred to as 32, an on-board controlunit (OCU), generally referred to as 34, and a base recording station(BRS), generally referred to as 36. Preferably, the data connection 38between the OCU 34 and BRS 36 is preferably wireless, however, ahardwired, tethered connected 38 is also possible. Together, thesecomponents provide a telemetered data acquisition system 30 formonitoring and recording sensor data on head impacts. The installedenvironment for the system 32 need not always be a helmet, and can beadapted for use in various forms in helmets or headgear for sportsincluding football, hockey, soccer, lacrosse, wrestling, boxing andothers. The HMSS unit 32 can be comprised of various additional sensorsincluding displacement, load, pressure, acceleration, temperature, etc.In the current configuration, the HMSS 32 system is composed of multipleaccelerometers 14 as described in detail above.

In FIG. 7, the BRS 36 and OCU 34 are preferably specified to be activeduring all practice and game situations. For team or multiple userconfigurations, the BRS 36 is either a laptop or PC 40, which isserially linked to a receiver 42 with capability for simultaneoustransmission from up to 100 OCU transmitters 34. Calculations show thatat a data transfer rate of 19.2 kbps, with maximum 100 bytes ofinformation from each OCU 34 per impact, data from all 22 players on thefield at any one time in sports such as soccer or football could bedownloaded to the BRS 36 within 1 second. For single userconfigurations, the BRS 36 could be a stand-alone data-logger, or couldbe contained internally within the OCU 34, with plug in capability fordownloading of data and programming. Triggering conditions programmedinto the OCU 34 activate the transmitter/data collection system 30 andsend information to the BRS 36. Power is conserved by turning thetransmitter portion of the OCU 34 on only when an impact event occurs.For example, a minimum acceleration of 10 g's might be set as thetrigger. Each OCU 34 uniquely identifies a given helmet 20 in the fieldand encodes the information so that the BRS 36 can properly multiplexand decode information from multiple OCU's.

In accordance with the present invention, a miniature telemetry system30 is provided with a transmitter/receiver that preferably operates inthe 900 MHz range with a range of at least 150 m. Analog signals fromthe accelerometers 14 will be time-division multiplexed (TDM) fortransmission to the BRS. The size of the OCU 34 is specified to be nolarger than 5 cm long×2.5 cm high×2.5 cm wide, or the size of 2 small AAbatteries. The OCU 34 can be mounted at the base of the helmet 20 in therear just above the neckline without interfering with player motion andwithout creating an injury hazard. The OCU 34 must contain the battery,the transmitter, and signal conditioning for the accelerometers.

The preferred accelerometers 14 operate at 3.3 V, the amplifier boards15 power the accelerometers 14 and provide signal conditioning for theraw accelerometer signals with a 10 Hz high pass filter to eliminatestatic measurements (such as player shaking his head). The chips of theADXL193/278 accelerometers have a 400 Hz 2-pole Bessel filter on-board.An additional 3000 Hz low pass filter on the amplifier board reducedhigh frequency noise that might enter the circuit after theaccelerometer chip 15 and before the amplifier.

Details of the above system 30 set forth a preferred construction forcarrying out the present invention. Such a system 30 may be modified tosuit the needs of the particular application at hand, namely theenvironment of installation, required capacity, durability and cost.Such modified systems 30 are deemed to be within the scope of thepresent invention.

Acceleration data is collected and recording for each of theaccelerometers 14 in the system 30 as described above. This data must beprocessed for meaningful analysis. Specifically, in accordance with thepresent invention, the actual linear and rotational acceleration of thehead and the magnitude of the impact is estimated using the arrangementof single-axis accelerometers 14 in the system 30 as described above.

The data collected and recorded by the accelerometers is processedaccording to a novel algorithm of the present invention. The processingof the data with the novel algorithm of the present invention assumesthat: 1) the accelerometers 14 are placed at known locations around thesurface of the skull 16 of the head 12, as shown in FIG. 2; and 2) thesurface of the skull 16 of the head 12 can be described geometrically.

For example, the novel algorithm can be demonstrated for a typical casewhere, in addition to the above assumptions, the following conditionsare met: 1) the accelerometers 14 are placed at known locations aroundthe transverse plane of the skull 16 of the head 12 passing through apoint 18 located approximate to the center of gravity, as shown in FIG.2; 2) the head cross-section (HCS) in this transverse plane is circular,and defines a radial coordinate system, as shown in FIG. 3; and 3) theimpact is linear and lies within the transverse plane.

For these conditions, it can be shown that the magnitude of the linearacceleration normal to the HCS varies as the cosine of the arc (s) alongthe HCS. A Hit Profile is defined by the following function:

a*cos(s−b)+c  (1)

where a=peak linear head acceleration (g's), s=arc (deg), b=hit locationon the head (deg) and c=the offset. For a given impact and a specificconfiguration of accelerometers 14, i.e. the number and location ofaccelerometers 14, there will be a set of n acceleration profiles andpeak accelerations. Given the location of each accelerometer, indegrees, in the HCS, a least-squares fit of the acceleration data to theHit Profile yields the predicted peak linear head acceleration, a, andthe predicted hit location, b, in the HCS. In the case where the impactis directed to the center of gravity of the head 12, the offset will bezero. Otherwise, as will be described below, axial rotational headacceleration will result requiring an offset value.

In general, the acceleration data is collected and recorded. A hitprofile function is determined from the configuration of the body partand the positioning of the plurality of accelerometers thereabout. Anumber of potential hit results are generated from the hit profilefunction and then compared to the acceleration data sensed by theaccelerometers. One of the potential hit results is best fit matched tothe acceleration data to determine a best fit hit result. The magnitudeand direction of an impact to the body part is determined from applyingthe hit profile function to the best fit hit result. The rotationalacceleration of the body part can also be determined from the magnitudeand direction of the impact to the body part and the offset.

EXAMPLE OF APPLICATION OF ALGORITHM

As shown in FIG. 8, the acceleration data for a given array of threeaccelerometers is graphically displayed in two dimensions. In thisexample, the accelerometers are placed at the known locations of (−)120degrees, 0 degrees and 120 degrees about the assumed circularcircumference of the skull of a head with a known arc length s which isthe radius r in FIG. 2. In this example, the accelerometers revealed animpact by sensing the following accelerations:

TABLE 1 Location of Accelerometer Peak Acceleration in Coordinate SystemSensed (g) (−) 120 75    0  8   120 75

These known parameters of the location of the accelerometers are used tocreate series of cosine waves from the above algorithm function whichare each slightly different than one another. This series of waveformscorrespond to the various potential hit magnitudes and hit locationscalculated using Equation 1. These waveforms are considered potentialhit results. As shown in FIG. 8, the series of waveforms 44 are mappedover the actual collected data 46. One of the waveforms 44 is selectedas a best fit hit result by employing known least squares regressiontechniques. The non-selected waveforms are discarded. The selected bestfit hit result, a cosine wave, is governed by the algorithm functionabove. Therefore, the additional variables of peak linear acceleration aand the hit location b in degrees can be determined by simply viewingthe particular mathematical components of the selected best fit result.Thus, the magnitude of the linear acceleration and direction of impactcan be calculated using only single-axis accelerometers.

The function above is employed when the HCS is assumed to be circular.Other functions are employed when the HCS is assumed to be other shapes,such as an ellipse. For an ellipse, the cosine wave hit profile ismodified by multiplication of the tangent of the ellipse and by divisionof the tangent of a circle. Using a similar approach, the function forany geometric shape can be employed to generate the hit profile for aparticular body part shape.

Further, rotational acceleration is also capable of being estimated fromthe linear data obtained from the single-axis accelerometers 14 and theestimation of the magnitude of acceleration and direction of impact.Specifically, In the case of impacts that are not directed towards thecenter of gravity, as shown in FIG. 2, an axial rotational accelerationis assumed to be induced about the z-axis, parallel to the spine throughthe neck or in the superior-inferior direction and through the center ofgravity 18 of the head 12 The normal component of this rotationalacceleration will be recorded by the linear accelerometers according tothe following function:

a _(n) =rω ²  (2)

where r is the distance from the z-axis passing through center ofgravity of the head 12 to the accelerometers 14 and ω is the angularvelocity of the head 12. In this case, the algorithm for fitting thelinear acceleration data to the cosine algorithm above worksequivalently and accounts for the offset in linear acceleration data dueto the normal component of angular acceleration. This offset definesaxial rotational acceleration about the z-axis— and is one of the threecomponents that completely describe the rotational acceleration of theskull. Thus, the rotational acceleration appears in the function informula (1) above as the offset and can be easily determined from theselected best fit curve. The antero-posterior and medial-lateral bendingacceleration of the skull are computed together by multiplying theestimated linear acceleration by the distance to the center of rotationof the neck for the given impact direction. This distance can be fixedfor all impact directions, selected from a lookup table, or measuredempirically. The estimate of the magnitude of the rotationalacceleration of the skull is given as the magnitude of the axial,antero-posterior and medial-lateral bending acceleration of the skull

Therefore, a further novel aspect of the system and method of thepresent invention is that computation of rotational acceleration isbased on the impact location. Such a computation is made even withoutthe assumption of orthogonality of the accelerometers relative to eachother and computation of the impact vector using the fitting algorithmdescribed above to collected data all using only single-axisaccelerometers orthogonal to the surface of a body part.

The algorithm set forth above in formula (1) has been validated bycomparison to theoretical and experimental data. The known inputswere: 1) number of accelerometers; 2) location on the transverse planeof the head of each accelerometer (measured in degrees), and, 3)magnitude (g's) and location (degrees) of the impact in the HCS. Tovalidate the algorithm, a sensitivity analysis of the independentvariables was performed. For a given set of these input variables, thecorrect (ideal) accelerations were calculated. To simulate variabilitythat would be expected in practical applications of system 30, randomnoise was added to the location of the accelerometers 14 and to theacceleration values. The algorithm used this noisy data set (repeated 10times for each parametric set of input variables) to predict themagnitude and location of the simulated hit. These values were thencompared to the input (ideal) values. Parametric analyses were performedby changing the number of accelerometers 14, the location of eachaccelerometer 14 location, the standard deviation of the noise in thelocation of the accelerometers, and the standard deviation of the noisein the peak acceleration values of each accelerometer.

Sensitivity analyses showed that computed values for peak linear headacceleration and hit location were most sensitive to errors inaccelerometer location compared to errors in acceleration magnitude.Table 2 below summarizes the effect on both estimated accelerationparameters and on commercial factors including cost and practicalimplementation.

TABLE 2 Effect on Effect on Decreasing Decreasing Error in Error inEstimated Estimated Effect on Peak Impact Effect Practical AccelerationLocation on Implementation Compared Compared System of System Parameterto Actual to Actual Cost in Helmets Increased ++ ++ + + HMAS MeasurementAccuracy Increased ++++ ++++ + +++ HMAS Location Accuracy Increased ++++++ +++ ++++ Number of HMAS Units

A configuration with 3 accelerometers spaced equally around thecoordinate system of FIG. 3 at 120° was sufficient, as shown in FIG. 9,to achieve errors in acceleration magnitude of less than 10%. From apractical perspective, a 3 accelerometer system, with positions at 0°,120°, −120° (0° was chosen as rear of the head, negative as left sideand positive as right side from a rear view of the head as in FIG. 3),demonstrated minimum error in peak acceleration predicted with noisyacceleration data compared to the actual (ideal) input peak accelerationand impact location across all impact locations on the transverse plane.Maximum error was less than 10%. Accuracy did not begin to fall offsubstantially until the 3 accelerometers were within 30 degrees of oneanother. There was also only slight decrease in accuracy forasymmetrical accelerometer placements, such as 0°, 90°, −45°, which maybe a more practical position for the units to be placed in the helmet.For brevity, the full parametric analysis is not reported.

Increasing from three accelerometers to six accelerometers resulted in anegligible increase in the accuracy of the estimated peak accelerationand estimated impact location for a given accelerometer configuration.

Increasing the number of accelerometers decreased error in estimatedpeak acceleration and impact location error for 30 g impact simulations(n=10) when the system variables accelerometer acceleration andaccelerometer location were perturbed with random noise of 5% and 5degrees, respectively.

For any single simulation at any hit location, the error did not exceed10% or 10 degrees. It is concluded that as long as the accelerometer isaccurate to within 5% and its location is known within 5 degrees, thereis no substantial benefit to increasing the number of accelerometersfrom three to six. The three accelerometer configuration is preferredfrom a cost and data management perspective, and meets the desiredspecifications.

Experimental Testing

Laboratory testing with a three accelerometer configuration demonstratedthat linear accelerations computed from the measured accelerometeraccelerations were within 10% for impacts in the transverse plane whencompared to an accelerometer at the center of gravity of the headform.Impact location was computed to be within 10° of the actual value.Estimates of rotational accelerations using linear accelerometers werewithin 10% of computed values using video and direct measurementtechniques.

A standard twin-wire drop system (ASTM F1446) was utilized for linearacceleration testing with a triaxial accelerometer mounted at the centerof gravity of a standard ISO headform. Peak acceleration from each ofthe three accelerometers was used as input for estimating the linearacceleration using the least squares fit algorithm described above.Actual accelerometer locations were measured using a laser protractorsystem. Five impacts at an impact velocity of approximately 2.2 m/s wererecorded at 45° intervals around the transverse plane of the headform.Computed peak acceleration data were compared with linear accelerationsmeasured by a triaxial accelerometer located at the center of gravity ofthe headform.

A separate guided drop tower (not shown) with free 2D rotation wasutilized to compare measured linear and rotational accelerations fromboth accelerometers and triaxial accelerometer at the center of gravityof the headform with 2D rotational accelerations?? measured using amagnetohydrodynamic rotational velocity sensor, such as the ARS-01 fromPhoenix, Ariz., and computed from a 2D high speed digital video system,such as Redlakes MotionScope (2000 Hz). Accelerations measured by theaccelerometers and by the triaxial accelerometer are a combination oflinear acceleration and the normal component of the rotationalacceleration.

The normal component: a_(n)=rω², can then be solved for ω anddifferentiated to determine the rotational acceleration. Alternatively,the tangential component: at a_(t)=rα, can be solved directly for α, therotational acceleration. We assume that the head and neck acts as arigid body during the impact. The radius, r, was the distance from thepivot point on the experimental apparatus and the center of gravity ofthe headform. Error analysis was performed by comparing 2D rotationalaccelerations estimated from our system with the calculated rotationalaccelerations from the high-speed video and the ARS sensor. For example,for a 2.2 m/sec drop, rotational accelerations on the order of 2000rad/sec² were measured from the video, and compared with an estimated1900 rad/sec² from the linear accelerometers, representing approximately5% difference.

Thus, the algorithm in accordance with the present invention wasvalidated by demonstrating that the error in estimated peak accelerationand estimated impact location was within ±10% of actual (ideal) when thesystem variables accelerometer acceleration and accelerometer locationwere perturbed with random noise of 5% and 5 degrees, respectively. Thestandard error bars, shown in FIG. 9, illustrate variability with 10simulations.

Estimates of linear and rotational acceleration from experimental datacollected with the system 30 were within ±10% of peak accelerationcompared to acceleration measurements taken at the center of gravity ofthe test headform. Reproducibility of the system was within ±5%.

As shown above, the algorithm for estimating linear and rotationalacceleration and magnitude has been validated for 2D and for impactsalong the transverse plane. In accordance with the present invention,the algorithm can be readily modified to 3D and tested boththeoretically and experimentally.

Therefore, the present invention provides for single axis accelerometersto be incorporated into an helmet such that the accelerometer is inapposition to the surface of the head and can worn by a user. Dual andtri-axis accelerometers may also be used to collect and recordadditional information, such as acceleration tangent to the surface ofthe skull, for further analysis and study.

The system 30 of the present invention enables the relationship betweenbiomechanical measures of linear and rotational acceleration and theclinically determined incidence of MTBI across demographic groups to bequantified, with a particular emphasis on children and youth in sports.The system 30 is capable of automatic monitoring of impact incidence andwill provide a basis for testing hypotheses relating impact severity andhistory to MTBI.

It would be appreciated by those skilled in the art that various changesand modifications can be made to the illustrated embodiments withoutdeparting from the spirit of the present invention. All suchmodifications and changes are intended to be covered by the appendedclaims.

What is claimed is:
 1. A device for monitoring the acceleration of abody part having an outer surface, comprising: a plurality of sensingdevices constructed and arranged orthogonal to the outer surface of thebody part and not orthogonal to each other to respectively detectacceleration in a corresponding plurality of directions which are eachorthogonal to the outer surface of the body part; the plurality ofsensing devices being constructed and arranged to generate a signal inresponse to a sensed acceleration in each of the corresponding pluralityof directions; a processing device connected to the plurality of sensingdevices and being constructed and arranged to receive signals from theplurality of sensing devices and determine the magnitude and directionof an impact to the body part in the plurality of directions which areeach orthogonal to the outer surface of the outer surface of the bodypart.
 2. The device of claim 1, wherein said plurality of sensingdevices are single-axis linear accelerometers.
 3. The device of claim 1,wherein said plurality of sensing devices are multi-axis linearaccelerometers with at least one axis thereof being orthogonal to theouter surface of the body part.
 4. The device of claim 1, furthercomprising: a protective layer of material positioned about the bodypart; and a plurality of portions of cushioning material disposedbetween the body part and the protective layer of material.
 5. Thedevice of claim 4, further comprising: a carrier web being closelyfitted around the body part; the plurality of sensing devices beingattached to the carrier web and positioned orthogonal and proximal tothe outer surface of the body part and not orthogonal to each other torespectively sense acceleration in directions which are each orthogonalto the outer surface of the body part and not orthogonal to each other.6. The device of claim 5, further comprising: a plurality of carrierclips positioned between the plurality of portions of cushioningmaterial; the carrier clips respectively carrying the plurality ofsensing devices and being positioned orthogonal and proximal to theouter surface of the body part and not orthogonal to each other torespectively sense acceleration in directions which are each orthogonalto the outer surface of the body part and not orthogonal to each other.7. The device of claim 5, wherein the plurality of sensing devices areembedded within the plurality at portions of cushioning material and arepositioned orthogonal and proximal to the outer surface of the body partand not orthogonal to each other to respectively sense acceleration indirections which are each orthogonal to the outer surface of the bodypart and not orthogonal to each other.
 8. The device of claim 1, whereinthe plurality of sensing devices are three devices positionedapproximately 120 degrees apart from one another about the circumferenceof the body part to respectively sense acceleration in three directionsresiding the same plane which are each orthogonal to the outer surfaceof the body part and not orthogonal to each other.
 9. The device ofclaim 1, further comprising: a recording station connected to theplurality of sensing devices.
 10. The device of claim 9, wherein therecording station is connected to the plurality of sensing devices bywire.
 11. The device of claim 9, wherein the recording station isconnected to the plurality of sensing devices by radio transmission. 12.The device of claim 1, wherein the body part is a head.
 13. The deviceof claim 1, wherein said plurality of sensing devices are mounted in ahelmet.
 14. The device of claim 1, wherein said plurality of sensingdevices are mounted in a head band.
 15. A method for determining themagnitude and direction of impact to a body part having a geometricshape, comprising the steps of: positioning a plurality ofaccelerometers proximate to the outer surface of a body part; orientingthe plurality of accelerometers to sense respective linear accelerationin respective directions which are each orthogonal to the outer surfaceof the body part and not orthogonal to each other; positioning theplurality of accelerometers in a defined arrangement about the surfaceof the body part; recording acceleration data sensed by the plurality ofaccelerometers in the directions which are each orthogonal to the outersurface of the body part and not orthogonal to each other; providing ahit profile function from the geometric shape of the body part and thepositioning of the plurality of accelerometers thereabout torespectively sense acceleration in directions which are each orthogonalto the outer surface of the body part and not orthogonal to each other;generating a plurality of potential hit results from the hit profilefunction; comparing the plurality of potential hit results to theacceleration data sensed by the plurality of accelerometers; best fitmatching one of the potential hit results to the acceleration data todetermine a best fit hit result; and determining the magnitude of linearacceleration, in each of the directions which are orthogonal to theouter surface of the body part and not orthogonal to each other, and thedirection of an impact to the body part from the best fit hit result.16. The method of claim 15, wherein the surface of the body part isdefined by the circumference of the body part, along which the pluralityof accelerometers is approximately a circle.
 17. The method of claim 16,wherein the hit profile function is equal to a*cos(s−b)+c where a is theimpact magnitude, s is the arc defining the accelerometer position, b isthe impact direction and c is the radial acceleration due to purerotation about the superior-inferior Z-axis and known data ranges areemployed to generate potential hit results.
 18. The method of claim 15,wherein the configuration of the body part is a geometric shape.
 19. Themethod of claim 18, wherein the configuration of the profile generatingfunction corresponds to the geometric shape.
 20. The method of claim 15,wherein matching one of the potential hit profiles to the accelerationdata employs least-squares regression model to determine the best fitprofile.
 21. The method of claim 15, further comprising: estimating therotational acceleration of the body part from the magnitude of linearacceleration in each of the directions which are orthogonal to the outersurface of the body part and not orthogonal to each other and thelocation of the impact to the body part from the best fit hit result bymultiplying the distance from the location of the impact to an axis ofrotation of the body part by the magnitude of the linear acceleration ofthe body part.
 22. A method of acceleration monitoring, comprising thesteps of: attaching an acceleration-monitoring technology device, havingacceleration sensors, to an individual such that the accelerationsensors remain fixed relative to a body part of the individual duringphysical activity where the body part has an outer surface; measuringaccelerations of the body part of the individual during physicalactivity along at least a first, a second and a third accelerationmeasurement direction, wherein the first acceleration measurementdirection is orthogonal to the outer surface of the body part, and thesecond acceleration measurement direction is orthogonal to the outersurface of the body part, and the third acceleration measurementdirection is orthogonal to the outer surface of the body part; the firstacceleration measurement direction, the second acceleration measurementdirection and the third acceleration measurement direction not beingorthogonal to each other; storing the accelerations of the body part ofthe individual in each of the first, second and third accelerationmeasurement directions which are each orthogonal to the outer surface ofthe body part and not orthogonal to each other, during the physicalactivity as acceleration data in a mass storage device; retrieving theacceleration data of the body part of the individual during physicalactivity; determining a direction and magnitude of the impact to thebody part of the individual during the physical activity and therotational acceleration of the body part of the individual during thephysical activity from the acceleration data.
 23. A device formonitoring the acceleration of a body part having an outer surface,comprising: a plurality of sensing devices constructed and arrangednon-orthogonally to each other to respectively detect acceleration in acorresponding plurality of directions; the plurality of sensing devicesbeing constructed and arranged to generate a signal in response to asensed acceleration in each of the corresponding plurality ofdirections; a processing device connected to the plurality of sensingdevices and being constructed and arranged to receive signals from theplurality of sensing devices and determine the magnitude and directionof an impact to the body part in the plurality of directions which arenot orthogonal to each other.
 24. The device of claim 23, wherein saidplurality of sensing devices are single-axis linear accelerometers. 25.The device of claim 23, wherein said plurality of sensing devices aremulti-axis linear accelerometers with plurality of axes that are notorthogonal to each other.
 26. The device of claim 23, furthercomprising: a protective layer of material positioned about the bodypart; and a plurality of portions of cushioning material disposedbetween the body part and the protective layer of material.
 27. Thedevice of claim 26, further comprising: a carrier web being closelyfitted around the body part; the plurality of sensing devices beingattached to the carrier web and not orthogonal to each other torespectively sense acceleration in directions which are not orthogonalto each other.
 28. The device of claim 27, further comprising: aplurality of carrier clips positioned between the plurality of portionsof cushioning material; the carrier clips respectively carrying theplurality of sensing devices and not being positioned orthogonal to eachother to respectively sense acceleration in directions which are notorthogonal to each other.
 29. The device of claim 27, wherein theplurality of sensing devices are embedded within the plurality ofportions of cushioning material and are not orthogonal to each other torespectively sense acceleration in directions which are not orthogonalto each other.
 30. The device of claim 23, wherein the plurality ofsensing devices are three devices positioned approximately 120 degreesapart from one another about the circumference of the body part torespectively sense acceleration in three directions residing the sameplane which are not orthogonal to each other.
 31. The device of claim23, further comprising: a recording station connected to the pluralityof sensing devices.
 32. The device of claim 31, wherein the recordingstation is connected to the plurality of sensing devices by wire. 33.The device of claim 31, wherein the recording station is connected tothe plurality of sensing devices by radio transmission.
 34. The deviceof claim 23, wherein the body part is a head.
 35. The device of claim23, wherein said plurality of sensing devices are mounted in a helmet.36. The device of claim 23, wherein said plurality of sensing devicesare mounted in a head band.
 37. A method for determining the magnitudeand direction of impact to a body part having a geometric shape,comprising the steps of: positioning a plurality of accelerometersproximate to the outer surface of a body part; orienting the pluralityof accelerometers to sense respective linear acceleration in respectivedirections which are not orthogonal to each other; positioning theplurality of accelerometers in a defined arrangement about the surfaceof the body part; recording acceleration data sensed by the plurality ofaccelerometers in the directions which are not orthogonal to each other;providing a hit profile function from the geometric shape of the bodypart and the positioning of the plurality of accelerometers thereaboutto respectively sense acceleration in directions which are notorthogonal to each other; generating a plurality of potential hitresults from the hit profile function; comparing the plurality ofpotential hit results to the acceleration data sensed by the pluralityof accelerometers; best fit matching one of the potential hit results tothe acceleration data to determine a best fit hit result; anddetermining the magnitude of linear acceleration, in each of thedirections which are not orthogonal to each other, and the direction ofan impact to the body part from the best fit hit result.
 38. The methodof claim 37, wherein the surface of the body part is defined by thecircumference of the body part, along which the plurality ofaccelerometers is approximately a circle.
 39. The method of claim 38,wherein the hit profile function is equal to a*cos(s−b)+c where a is theimpact magnitude, s is the arc defining the accelerometer position, b isthe impact direction and c is the radial acceleration due to purerotation about the superior-inferior Z-axis and known data ranges areemployed to generate potential hit results.
 40. The method of claim 37,wherein the configuration of the body part is a geometric shape.
 41. Themethod of claim 40, wherein the configuration of the profile generatingfunction corresponds to the geometric shape.
 42. The method of claim 37,wherein matching one of the potential hit profiles to the accelerationdata employs least-squares regression model to determine the best fitprofile.
 43. The method of claim 37, further comprising: estimating therotational acceleration of the body part from the magnitude of linearacceleration in each of the directions which are not orthogonal to eachother and the location of the impact to the body part from the best fithit result by multiplying the distance from the location of the impactto an axis of rotation of the body part by the magnitude of the linearacceleration of the body part.
 44. A method of acceleration monitoring,comprising the steps of: attaching an acceleration-monitoring technologydevice, having acceleration sensors, to an individual such that theacceleration sensors remain fixed relative to a body part of theindividual during physical activity where the body part has an outersurface; measuring accelerations of the body part of the individualduring physical activity along at least a first, a second and a thirdacceleration measurement direction, wherein the first accelerationmeasurement direction, the second acceleration measurement direction andthe third acceleration measurement direction are not orthogonal to eachother; storing the accelerations of the body part of the individual ineach of the first, second and third acceleration measurement directionswhich are not orthogonal to each other, during the physical activity asacceleration data in a mass storage device; retrieving the accelerationdata of the body part of the individual during physical activity;determining a direction and magnitude of the impact to the body part ofthe individual during the physical activity and the rotationalacceleration of the body part of the individual during the physicalactivity from the acceleration data.